Ohm
The ohm (symbol: Ω) is the unit of or, in the degenerate case, . Definition An ohm is a resistance that produces a potential difference of one when a current of one is flowing through it. butt :1 Ω = 1 V/A = 1 m2•kg•s–3•A–2 Origin The ohm is named after , a , who discovered the relation between and , expressed in . Explanation By definition from , a has a resistance of one ohm if a voltage of one volt causes a current of one ampere to flow (R = V/I). Alternatively and equivalently, a device that dissipates one watt of power with one ampere of current flowing through it has a resistance of one ohm (R = P / I 2). Since , the ohm has been maintained internationally using the , where a conventional value is used for the ' constant', fixed by the 18th as R{K-90} = 25812.807 Ω. The complex number quantity is a generalization of resistance. Its real part is resistance and its imaginary part is . Impedance, resistance and reactance all have units of ohms. The symbol for the ohm is the Greek alphabet capital letter omega (letter) (Ω). If the Greek letter cannot be used, the word ohm is used instead. The various guides for the use of the International System of Units do not explicitly forbid the elision of the final "o" of some SI prefixes, although there is nothing in them to suggest that it is allowable, either. As a result, one is just about as likely to see "kilohm", "kiloohm" and even "kilo-ohm", and the same holds true for hecto-, micro-, nano-, pico-, femto-, atto-, zepto-, and yocto-. The only other SI unit to suffer from this kind of orthographic uncertainty is the . In the particular case of the ohm, one even sees the "a" prefixes lose that vowel: hence megohm and gigohm. Higher prefixes are rarely used with ohm. In the other direction, milliohms (or millohms) are seen where the resistance of cables, etc., are measured. Units of ohms, kilohms (103 Ω) and megohms (106 Ω) are used in electronic design documentation. On schematic diagrams kilohms are abbreviated "K" and megohms are abbreviated "M". Thus, 33 kilohms would be rendered as 33K, and 5.1 megohms would be 5.1M. Values less than 1K are rendered without any symbol following the number, so 680 ohms would simply be shown as 680. This does not cause confusion, because the numeric value is placed next to a schematic symbol for a resistor, and the resistor is usually identified by a reference designator, R, plus a numeric part, e.g., R12. Conversions A measurement in ohms is the reciprocal of a measurement in , the SI unit of . Note that 'siemens' is both and . The reciprocal of the ohm is also called the , from ohm written backwards. See also * * External links *Scanned books of Georg Simon Ohm at the library of the University of Applied Sciences Nuremberg * Official SI brochure * NIST Special Publication 811 * Ohm's Law Calculator Category:SI derived units Category:Units of electric resistance or electrical impedance